我使用的是 ffmpeg 4.2.2在Ubuntu 20.04机器上克隆一个USB网络摄像头的信号源(v4l2 器件 /dev/video0)，以便多个应用程序可以访问feed。我使用了这个 ffmpeg 命令，效果很好。
ffmpeg -f v4l2 -i /dev/video0 -codec copy -f v4l2 /dev/video1
本质上，这使用了一个 v4l2 回环器 /dev/video1 而消费型应用只是将其视为一个普通的 v4l2 设备。
在这之后，我遇到了需要改变输出像素格式的问题（相关的 所以问题). 我的源设备提供 yuv420p 而水槽应用只能处理 yuyv422. 为了达到这个目的，我使用了 -pix_fmt 参数。然而，当使用 -codec copy 所以我需要明确地指定编解码器。这不是问题，因为 v4l2 设备只支持原始视频(至少据我所知).因此，我的结局是这样的。
ffmpeg -f v4l2 -i /dev/video0 -c:v rawvideo -pix_fmt yuyv422 -f v4l2 /dev/video1
不幸的是，这似乎很不稳定。在启动这个命令的10次中，大约有1次能按预期工作。ffmpeg 开始克隆feed并转换颜色格式。
joel@joel-ubuntu:~$ ffmpeg -f v4l2 -i /dev/video0 -c:v rawvideo -pix_fmt yuyv422 -f v4l2 /dev/video1
ffmpeg version 4.2.2-1ubuntu1 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 9 (Ubuntu 9.3.0-3ubuntu1)
configuration: --prefix=/usr --extra-version=1ubuntu1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-nvenc --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
libavutil 56. 31.100 / 56. 31.100
libavcodec 58. 54.100 / 58. 54.100
libavformat 58. 29.100 / 58. 29.100
libavdevice 58. 8.100 / 58. 8.100
libavfilter 7. 57.100 / 7. 57.100
libavresample 4. 0. 0 / 4. 0. 0
libswscale 5. 5.100 / 5. 5.100
libswresample 3. 5.100 / 3. 5.100
libpostproc 55. 5.100 / 55. 5.100
[video4linux2,v4l2 @ 0x55a4f38a5700] Time per frame unknown
Input #0, video4linux2,v4l2, from '/dev/video0':
Duration: N/A, start: 11224.864514, bitrate: N/A
Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 640x480, 14.33 tbr, 1000k tbn, 1000k tbc
Stream mapping:
Stream #0:0 -> #0:0 (rawvideo (native) -> rawvideo (native))
Press [q] to stop, [?] for help
Output #0, video4linux2,v4l2, to '/dev/video1':
Metadata:
encoder : Lavf58.29.100
Stream #0:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 640x480, q=2-31, 70451 kb/s, 14.33 fps, 14.33 tbn, 14.33 tbc
Metadata:
encoder : Lavc58.54.100 rawvideo
frame= 214 fps= 15 q=-0.0 Lsize=N/A time=00:00:14.93 bitrate=N/A dup=1 drop=0 speed=1.05x
video:128400kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Exiting normally, received signal 2.
然而，其他时候，我得到的是非常陈旧的行为 除了第一帧之外，输出永远无法使用（然后它就冻结了），而且... ... ffmpeg 在短短的几秒钟内就出现了超过10万帧的重复帧数，同时报告的FPS也超过了3万。几秒钟后，它就崩溃了，或者干脆就挂了，直到我。Ctrl^C 它。
joel@joel-ubuntu:~$ ffmpeg -f v4l2 -i /dev/video0 -c:v rawvideo -pix_fmt yuyv422 -f v4l2 /dev/video1
ffmpeg version 4.2.2-1ubuntu1 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 9 (Ubuntu 9.3.0-3ubuntu1)
configuration: --prefix=/usr --extra-version=1ubuntu1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-nvenc --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
libavutil 56. 31.100 / 56. 31.100
libavcodec 58. 54.100 / 58. 54.100
libavformat 58. 29.100 / 58. 29.100
libavdevice 58. 8.100 / 58. 8.100
libavfilter 7. 57.100 / 7. 57.100
libavresample 4. 0. 0 / 4. 0. 0
libswscale 5. 5.100 / 5. 5.100
libswresample 3. 5.100 / 3. 5.100
libpostproc 55. 5.100 / 55. 5.100
[video4linux2,v4l2 @ 0x561ef693c700] Time per frame unknown
Input #0, video4linux2,v4l2, from '/dev/video0':
Duration: N/A, start: 11156.178415, bitrate: N/A
Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 640x480, 1000k tbr, 1000k tbn, 1000k tbc
Stream mapping:
Stream #0:0 -> #0:0 (rawvideo (native) -> rawvideo (native))
Press [q] to stop, [?] for help
[video4linux2,v4l2 @ 0x561ef6940400] Frame rate very high for a muxer not efficiently supporting it.
Please consider specifying a lower framerate, a different muxer or -vsync 2
Output #0, video4linux2,v4l2, to '/dev/video1':
Metadata:
encoder : Lavf58.29.100
Stream #0:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 640x480, q=2-31, 4915200000 kb/s, 1000k fps, 1000k tbn, 1000k tbc
Metadata:
encoder : Lavc58.54.100 rawvideo
More than 1000 frames duplicated
More than 10000 frames duplicated
More than 100000 frames duplicated=N/A time=00:00:00.15 bitrate=N/A dup=151050 drop=0 speed=0.0456x
27653321 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=0 speed=0.0289x
27733577 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=1 speed=0.0256x
27787315 frame duplication too large, skipping
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frame=707641 fps=20090 q=-0.0 Lsize=N/A time=00:00:00.70 bitrate=N/A dup=707630 drop=129 speed=0.0201x
video:424584600kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Exiting normally, received signal 2.
我不知道是什么原因导致了这种行为 也不知道如何追踪和解决它。是否有人有经验与这种类型的行为或有一个简单的问题在我的设置？
注：我使用的是DroidCam。DroidCam提供了一個Linux客戶端，而這個客戶端又提供了一個新的客戶端。v4l2 输出设备。除了一些小问题之外，我没有注意到任何与普通网络摄像头不同的行为。不幸的是，我无法用普通的USB UVC网络摄像头重现这种情况，因为网络摄像头已经提供了所需的输出像素格式。这种输出像素格式的转换其实只是必要的，因为 DroidCam 产出 yuv420p 但消耗流的应用程序只能处理 yuyv422.
V4L2设备是一个恒定的帧率写入器，所以如果源帧率较低，ffmpeg会重复帧以保持恒定的帧率。在没有用户设置帧率的情况下，ffmpeg将其设置为输入的帧率。对于没有明确发出帧率信号的采集设备，ffmpeg会通过寻找帧之间的最小间隔来推断帧率。
当ffmpeg检测到 1000k tbr，这时你就会得到大规模的重复。要么设置一个合理的数字 -r 15 或禁用帧复制。-vsync vfr.